Electrical oscillation generator



March 9, 1954 A CQRMACK 2,671,856

ELECTRICAL OSCILLATION GENERATOR Filed Nov. 3, 1950 INVENTO'R ELF! ("o/w K FITTORNEY Patented Mar. 9, 1954 ELECTRICAL OSCILLATION GENERATOR Alan Cormack, Kenton, England, assignor to The General Electric Company Limited, London,

England Application November 3, 1950, Serial No. 193,900

Claims priority, application Great Britain November 22, 1949 6 Claims. 1

This invention relates to electrical oscillation generators and more particularly to generators of the kind which comprises in combination a first thermionic valv arranged as a resistancecapacity coupled amplifier stage and at least three thermionic valves each arranged as a cathode follower stage, the amplifier stage and the cathode follower stages being connected in a closed loop so that, during operation, the phase shift round the loop at the frequency of operation is such that the combination is self-oscillatory.

In one example of the generator of this kind there are provided four cathode follower stages and, when operating at frequencies at which the interelectrode capacity of the first valve has negligible effect, the amplifier stage gives a constant phase shift of 180 over a wide range of operating frequencies and th cathode follower stages are arranged each to give a 45 phase shift. For this purpose each cathode follower stage may have a condenser shunting a resistance in the cathode circuit of the valve and it can be shown that the phase shift through such a cathode follower stage at a given frequency is a function of the mutual conductance of the valve in that stage. Thus, sinc the amplifier stage introduces a fixed phase shift, the generator operates at the frequency which gives a phase shift of 45 in each cathode follower stage, the operating frequency being controlled merely by varying the mutual conductance, for example by simultaneously varying the anode currents, of the four cathode follower valves. Moreover it can be shown that under these conditions the amplitude of output oscillations from the generator is substantially independent of operating frequency over a wide range of frequencies. The upper limit of oper-- ating frequency is set by the maximum obtainable mutual conductance of the cathode follower valves.

It is one object of the present invention to provide a generator of the kind specified in which the upper limit of the useful range of operating frequencies is extended.

According to the present invention in an electrical oscillation generator of the kind specified, the anode load of the said first valve is arranged to be of variable impedance whereby the operating frequency, over at least part of the range of operating frequencies, of the generator may be controlled by varying the said impedance.

The anode load may comprise the effective impedance across the output of a further cathode follower stage, this eifective impedance being arranged to be varied by varying the mutual conductance of the valve in the further cathode follower stage. The anode load may consist of a resistance connected across the output of the further cathode follower stage.

The operating frequency of the generator may be varied over the lower part of the range by controlling the mutual conductance of the valves in the cathode follower stages and over the upper part of the range by controlling the mutual conductance of the valve in the further cathode follower stage. Controlling means may be provided to vary the operating frequency of the generator over its whole range in dependence upon the magnitude of a signal, the controlling means being arranged to vary the mutual conductances of the valves over both the lower and upper parts of the operating range. The lower and upper parts of the range may overlap slightly and preferably the arrangement is such that the generator has a substantially fiat amplitude characteristic over its whole frequency range. The controlling signal may be derived from a potentiometer or, if the output oscillations are required to sweep cyclically through a band of frequencies, a signal having a saw-tooth waveform may be applied to the controlling means. Other signals may of course be fed to the controlling means to produce any required frequency modulation of the output oscillations.

An electrical oscillation generator in accordance with the invention and which is adapted to produce oscillations of substantially constant amplitude at any frequency over a wide range of frequencies will now be described by way of example with reference to the accompanying diagrammatic drawing which shows the electrical circuit of the generator.

Referring to the drawing, the generator comprises a resistance-capacity coupled amplifier stage I and four cathode follower stages 2, 3, 4 and 5, all these stages I to 5 being connected in a closed loop so as to be self-oscillatory. The amplifier stage I has a triode thermionic valve 6 arranged with its cathode I connected to earth and its anode 8 connected through a load resistance ID to a first terminal I I which is maintained at a positive potential with respect to earth. Each cathode follower stage 2, 3, 4 or 5 has a triode thermionic valve I2 and a coupling condenser I3 is connected between the anode 8 of the amplifier valve 6 and the grid I4 of the valve I2 of the first cathode follower stage 2. The valve I2 in each cathode follower stage 2, 3, 4 or 5 is arranged with its anode I5 connected directly to a second terminal I6 which is arranged to be maintained at a potential positive with respect to earth and its cathode I! connected through an associated load resistance l3 to earth, the cathode I! of the valve l2 in each of the stages 2, 3, 4 and 5 being connected through a coupling condenser to the grid l4 of the next stage 3, 4 or 5 or the grid 2| of the amplifier valve -6. A leak resistance 22 is provided between the grid 2| and cathode of the amplifier valve 6 and a bias path is provided for th grids M of the other valves l2 as hereinafter described.

The output of the generator is taken from across the load resistance I8 of the cathode follower stage 2 through an additional cathode follower stage 23. This stage 23 includes a valve and acts as a buffer stage, the output oscillations being developed between the terminal 24 and earth.

It is desirable that the mutual conductance of these four valves l2 in the cathode follower stages 2 to 5 shall be as high as possible and that the interelectrode capacity between the grid l4 and cathode I! of each valve l2 shall be low. All the valves 6, l2 and 25 may be of the kind adapted for very high frequency operation having parallel planar electrodes and in one example using such valves the amplifier valve 6 has an anode load H] of 680 ohms and a leak resistance 22 of 33,000 ohms, the cathode load [8 in each of the cathode follower stages 2 to 5 is 8,200 ohms, each coupling condenser 20 is 1000 micromicrofarads and the terminals II and I6 are maintained at potentials of 150 and 250 volts respectively above earth.

With the generator as so far described, it is found that by simultaneously varying the steady bias voltage on the four cathode follower valves l2, and thereby varying the mutual conductance of the valves, the operating frequency of the generator may be varied over a wide range, for example between approximately 40 and 100 megacycles per second, the amplitude of the output oscillations being substantially constant over this range. In order to extend the useful operating range a further cathode follower stage 26 is arranged to couple a variable impedance which is mainly resistive across the anode load H] of the amplifier valve 6. This cathode follower stage 26 has a triode thermionic valve 21 which is of the kind previouslymentioned and is arranged with its anode 28 connected directly to the positive terminal its cathode 30 is connected to earth through a load resistance 3|, and

its grid 32 is connected to earth through a low resistance 33' and a blocking condenser 34. The effective impedance across the cathode load 3| of this stage 26 is coupled across the anode load I0 of the amplifier stage I by a coupling condenser connected between the cathode 30 of the valve 21 and the anode 8 of the valve 6. The interelectrode capacity between the grid 32 and cathode 30 of this stage 26 results in the output impedance coupled across the anode load In of the amplifier valve 6 having an inductive component and merely by varying the bias voltage on the grid 32 of the valve 21, that is to say by varying the mutual conductance of the valve 21, it is possible to vary the operating frequency of the generator over an extended range of frequencies, in one example between 100 and 150 megacycles per second, and by suitable choice of the values of the components the amplitude of the output oscillations may be substantially constant over this upper part of the whole range and the same as over the lower part of the range.

4 Preferred values for the components in this stage are 2,200 ohms for the cathode load 3|, ohms for the resistance 33, 1,000 micro-microfarads for the condenser 34 and 1,000 micro-microfarads for the coupling condenser 35.

Control of the operating frequency of the generator is effected by varying the magnitude of the unidirectional control voltage supplied over the line 35 and the frequency is at its maximum when the control voltage is a maximum. Switching between the upper and lower parts of the range is carried out by means of two diodes 38 and 40 provided by a double-diode thermionic valve 31. The anodes 4| and 42 of the diodes 38 and 48 are connected through resistances 43 and 44 respectively to the line 36, the cathode 45 of the diode 38-being connected to the junction of two series-connected resistances 46 and 4! which are connected between the positive terminal I6 and earth. Each grid l4 of the four valves I2 is connected through an associated resistance 48 to the anode 4| of the diode 38. The diode 40 is arranged with its cathode 50 connected to both the junction of the resistance 33 and blocking condenser 34 in the grid circuit of the valve 21 and through a high resistance 5| to a first negative terminal 52 maintained at a negative poten-' tial to earth. The anode 42 of this diode 40 is connected through a resistance 53 to a second negative terminal 51 maintained at a more negative potential to earth.

When the operating frequency is its minimum value, both diodes 38 and 40 are non-conducting and the valve 21 is biassed' beyond cut-off so that there is no impedance coupled across the anode load In of the amplifier valve 6. As the control voltage is increased, the operating frequency is increased by simultaneously increasing the bias voltage on the grids M of the four cathode follower valves |2. Increase of this bias voltage substantially beyond the value corresponding to the highest operating frequency of the lower part of the frequency range is prevented by the diode 38 of the double diode valve 31 conducting and at a slightly lower control voltage the second diode 40 becomes conducting whereby the cathode follower stage 26 is released so that further increase of the control voltage causes the operating frequency of the generator to be increased. by raising the bias voltage on the grid 32 of the valve 21.

By suitable choice of the values of the components there may be a substantially linear relationship between the voltage on the line 35 and the output frequency. Thus the abovementioned example, in order to provide the op-- erating range between 40 and 150 megacycles per second the bias voltage on the grids of the.

four cathode follower valves is required to be varied between +5 and volts with respect to earth and the bias voltage on the grid '32. of the valve 21 is to be varied between 30 and-1, 50

volts. For this purpose a control voltage which may be varied over the range of approximately.

+5 to +250 volts with respect to earth is necessary while the resistance 43 may have avalue of 3.3 meg'homs, the resistance 44 of 220,000, ohms, the resistance 5| is 1 meghom, the re sistance 53 of 220,000 ohms, while the negative terminals 52 and 5': are at potentials of 30 and 125 volts respectively with respect to earth two series connected resistances 54 and 55 connected between the positive terminal 16 and earth. Suitable values for these resistances 54 and 55, in order to provide the required control voltage are 50,000 ohms and 1,000 ohms respectively.

It will be appreciated that instead of deriving the control voltage from the said potentiometer and varying the operating frequency by manual movement of the tapping point 56, a voltage having a recurrent saw tooth, for example, waveform may be used so as to cause the generator to sweep periodically through its range of operating frequencies.

In the generator described above there are four cathode follower stages in the phase-shift path and it will be realised that, although this number is preferred to give a wide range of operating frequencies, three, four or more such stages may be employed.

A radio frequency choke may be connected in the heater circuit of each of the valves l2 of the cathode-follower stages 2 to 5 so as to reduce the capacity between each cathode I and earth. This modification results in an increase in the maximum operating frequency that can be obtained with the generator but, owing to instability at relatively low frequencies it also increases the minimum operating frequency.

I claim:

1. An electrical oscillation generator comprising an amplifier stage including a first thermionic valve of the kind having a control grid, at least three cathode follower stages each including a thermionic valve having a control grid, means to connect said cathode follower stages in cascade, means to feed the output from the last of these cascade-connected cathode follower stages to the control grid of the first thermionic valve, a further cathode follower stage including a thermionic valve of the kind having a control grid, means to connect the further cathode follower stage in circuit with the first thermionic valve so that the anode load impedance of that valve includes the output impedance of the further cathode follower stage, a capacitative couplings to feed an oscillatory signal developed across said anode load impedance to the control grid of said valve forming the first of the cascade-connected cathode follower stage, and frequency control means to supply a variable bias signal to the control grid of said valve forming the further cathode follower stage so as to vary the mutual conductance of that valve.

2. .An electrical oscillation generator according to claim 1 wherein a resistance is provided in the anode circuit of the first thermionic valve and this resistance is connected across the output of said further cathode follower stage.

3. An electrical oscillation generator which may be tuned over a range of operating frequencies comprising an amplifier stage including a first thermionic valve of the kind having a control grid, at least three cathode follower stages each including a thermionic valve having a control grid, means to connect said cathode follower stages in cascade, means to feed the output from the last of these cascade-connected cathode follower stages to the control grid of the first thermionic valve, a further cathode follower stage including a thermionic valve of the kind having a control grid, means to connect the further cathode follower stage in circuit with the first thermionic valve so that the anode load impedance of that valve includes the output impedance of the further cathode follower stage, a capacitative coupling to feed an oscillatory signal developed across said anode load impedance to the control grid of said valve form-. ing the first of the cascade-connected cathode follower stages, frequency control means for producing an electrical signal the magnitude of which is dependent upon the required operating frequency of the generator, and means responsive to the magnitude of said signal to supply a first bias signal to the control grids of the valves forming the cascade-connected cathode follower stages and a second bias signal to the control grid of the valve forming the further cathode follower stage so that the first bias signal is varied over a lower part of the frequency range and the second bias signal is varied over an upper part of the frequency range.

4. An electrical oscillation generator according to claim 3 wherein said means for supplying said first and second bias signals comprises first and second diode thermionic valves, first and second resistances connected between said frequency control means and the anodes of said first and second diode valves respectively, means for maintaining the cathode of said first diode valve at a positive potential, a first source of negative potential, a third resistance connected between the cathode of said second diode valve and the first source of negative potential, a second source of negative potential which is more negative than the first source, a fourth resistance connected between the anode of said second diode valve and the second source of negative potential, means for feeding the first bias voltage from the anode of the first diode valve, and means for feeding the second bias voltage from the cathode of the second diode valve.

5. An electrical oscillation generator comprising a first thermionic valve arranged as an amplifier stage, an anode load impedance for said amplifier stage, at least three cathode follower stages each including a thermionic valve having a control grid, means to connect said cathode follower stages in cascade, a capacitative coupling to feed an oscillatory signal developed across said anode load impedance to the first of the cascade-connected cathode follower stages, means to feed the output from the last of these cascade-connected cathode follower stages to the amplifier stage, first frequency control means to supply a variable bias signal to the control grids of the valves forming the cascade-connected cathode follower stages, and second frequency control means for varying said anode load impedance.

6. An electrical oscillation generator comprising a first thermionic valve arranged as an amplifier stage, an anode load impedance for said amplifier stage, at least three cathode follower stages each including a thermionic valve of the kind having a control grid, means to connect said cathode follower stages in cascade, resistance-capacity coupling between said amplifier stage and the first of the cascade-connected cathode follower stages and including a mainly resistive anode load impedance for said amplifier stage, means for feeding the output from the last of the cascade-connected cathode follower stages to the amplifier stage, first frequency control means to supply a bias signal to the control grids of the valves forming the cascade-connected cathode follower stages, second frequency control means to vary the anode load impedance of 1 8"- the amplifier stage, and means to vary the op-' References Cited'in the file of this patent erating frequency of the oscillation generator by UNITED STATES PATENTS controlling the first frequency control means over a lower part of the frequency range and Number Name Date the second control means over an upper part 5 2453073 Webb 1949 of theirequency range to give the generator a 2506329 Ames May 1950 substantially fiatamplitude characteristic as its 2,590,282 Thompson 1952 ALAN CORMACK.' 

